1. Field of the Invention
The present invention relates to a digital versatile disc (DVD) playback system. Specifically, it relates to a multimedia DVD playback system capable of playing two subtitles at the same time.
2. Description of the Related Art
Per U.S. Pat. No. 5,978,916, a DVD stores video, audio and compressed data in a multimedia storage format. A DVD decoder is utilized in the playback of video, audio and subtitles (hereinafter sub-picture) data files. In the case of video files, the encoded video data format conforms to the Moving Picture Experts Group (MPEG) specification.
Present DVD playback systems feature many playback capabilities. For example, users can select such functions as audio language, sub-title language, viewing angle, and aspect ratio. Moreover, DVD playback systems have additional functions such as video signal output or audio signal output.
In U.S. Pat. No. 5,907,658, a DVD playback system may display one subtitle from a list of options. To accommodate international audiences, one DVD title may include multiple subtitles in different languages with the DVD playback system providing the capability to access different subtitles based on locale or user defined options.
In FIG.1, the digital data read from a DVD is stored in a buffer memory (not shown). System parser 15 divides the digital data into video, audio, and sub-picture streams according to stream IDs using an eight digit binary flag format. Despite the existence of more than one stream for audio and sub-picture data, only one stream (per function) is used for decoding and output. The specified, functional data streams are then transferred to video decoder 2, a sub-picture decoder 6, and audio decoder 4 respectively. Audio decoder 4 decodes the selected audio stream and outputs the audio frames to audio output module 5 to generate acoustic sound/music output. Similarly, video decoder 2 decodes the video stream and generates video frames while sub-picture decoder 6 decodes the selected sub-picture stream and generates sub-picture frames. Both video and sub-picture frames are then transferred to and mixed by video/sub-picture composer 8. The mixed frames are then transferred to video output module 3 and displayed on-screen.
With this conventional DVD playback system, users can only choose a single sub-picture stream to be displayed at any given time. For example, sub-picture stream 0 is the English subtitle and sub-picture stream 1 is the French subtitle. When the user chooses the English subtitle, the sub-picture stream 0 (English) is automatically selected by system parser 15 and transferred to sub-picture decoder 6 for decoding. In this example, sub-picture stream 1 (French) would be discarded. The result being, only the English subtitle is displayed on-screen.
One disadvantage of conventional DVD playback systems is that even if a DVD title provides subtitles for different languages and the capability to select subtitle language, it can only display one subtitle at a time. Hence, conventional DVD playback systems do not/cannot meet the demands of a bilingual teaching environment, for example.
One objective of the present invention then is to provide a DVD playback system capable of playing two subtitles simultaneously. For example, a user may choose to display two different subtitles, one being the user""s native language and the other being a foreign language. The purpose being the user can cross-reference the two subtitles in an effort to learn the foreign language.
The present invention achieves the above objective by providing a method that allows DVD playback systems to play two subtitles simultaneously. Refer to FIG. 2. In the prior art, Navigator 20 does not accept two sub-picture streams concurrently. It decides on and selects one sub-picture stream ID corresponding to the title setting selected by the user and sends that ID to Loader System 32. The present invention instead provides a new, second control path (in addition to the original one controlled by Navigator 20) that allows the user to define other sub-picture stream IDs that can be directly assigned from User. Interface Control Unit 24 to Loader System 32. Loader System 32 in-turn directs System Parser 34 to deliver these user selected sub-picture streams to Sub-Picture Decoder 46 as defined by the two sub-picture stream IDs (e.g., two different sub-picture frames are decoded from the two sub-picture streams). The sub-picture corresponding to the first stream ID from Navigator 20 is called the Primary Sub-Picture and another sub-picture corresponding to the stream ID from User Interface Control Unit 34 is called the Secondary Sub-Picture. The Primary Sub-Picture and the Secondary Sub-Picture are first combined in a mixed sub-picture frame. The mixed sub-picture frame is then mixed with the video frame by Video/SP composer 48. The Video/SP mixed frame is then delivered to Video Output module 36 and displayed on-screen. Audio output proceeds concurrently.
In summary, the DVD playback system of the present invention is capable of playing two subtitles simultaneously and has the following features:
1. The end-user fully controls selection of the two subtitles including enable or disable as well as language(s).
2. The two subtitles will be displayed correctly and synchronously.
3. The playing of two subtitles will not effect or delay the playing of video or audio streams, thus be displayed in real-time.